The field of optics has had a long and unfilled need for a cost efficient and accurate system for measuring various properties of materials such as quartz, glass, and the like. One system has recently been discovered in which the coefficient of thermal expansion the (CTE) of optical glass may be measured accurately using apparatus and a method disclosed in my commonly owned application for A METHOD AND APPARATUS FOR MEASURING COEFFICIENT OF THERMAL EXPANSION, now U.S. Pat. No. 4,989,980.
The method disclosed in my patent includes the steps of generating an interference pattern defined by Newton rings which are a function of the specific material being examined. The area of at least one Newton ring is measured, and then the magnitude of change is the dimension is determined as a function of changes in area of that Newton ring. When temperature is used to change the dimension, a coefficient of thermal expansion can be calculated. A scale factor is determined which is a function of the difference between the area of a pair of successive Newton rings and of the wavelength of the laser beam.
The measurement of the area of the Newton rings is most difficult to measure precisely. These rings are not perfect circles with easy to measure areas. To more effectively use the apparatus in my patent, a method for calculating the area of an image, such as Newton rings which are produced by laser beam interference on a test sample was developed and is the subject of co-pending U.S. patent application for READOUT SYSTEM FOR DILATOMETERS, having Ser. No. 706,686, filed May 29, 1991 in the name of Hansen and commonly owned. The method includes a plurality of steps which permit computer operation to automate the procedure. Video images of Newton rings or fringes are taken, such as at various temperatures or under other conditions which might be varied to observe the changes in the test sample caused by the varied conditions. The area of the fringe or fringes is determined using a computer implemented algorithm.
Those new methods and apparatus described in the aforementioned patent and application are designed generally to measure test sample dimensional changes as a function of temperature so as to determine the coefficient of thermal expansion of optical materials. It would be of great advantage to the art if there would be a way to measure other important material properties beyond that of CTE.
One important property which is not presently measured directly in the optical field is the change in refractive index of a material with respect to the change in temperature of the material. Present methods measure indirect changes, by measuring changes in a bending angle with temperature. The prior art does not contain a method of measuring direct changes in the optical path length with respect to temperature. The prior art methods include measurements of angles with respect to temperature and have relatively low sensitivity. Thermal control is poor and therefore the method is much less accurate.
For that reason, it is an object of this invention to provide a method for directly measuring changes in refractive index of materials with respect to changes in temperature.
Another object is to employ new devices which have been developed for optical measurements and evaluation, such as my patent and the application described above, and to use these new methods to produce additional information such as change in refractive index with respect to temperature.
Other objects will appear hereinafter.